The invention relates generally to the field of telecommunications and more particularly to systems and methods for improving the efficiency of communications such as session initiation protocol messages for setting up voice-over-IP calls wherein these communications are transmitted over limited-bandwidth links, including wireless links between mobile stations and base stations.
Data can be communicated between devices coupled to a network using, e.g., IP addresses. Although, in the past, the data communicated was typically numeric or textual data, systems and methods have been developed for communicating voice data over these same networks. These communications are referred to generally herein as “voice-over-IP” communications.
In order to utilize voice-over-IP communications, it is necessary to set up a voice-over-IP call. Setup of the call is performed using what is known as the session initiation protocol (SIP). SIP is a call set up signaling protocol. SIP messages contain header information which is necessary to set up the call. The payload corresponding to these headers is communicated using the session description protocol (SDP).
Typically, a node that wishes to initiate a voice-over-IP call transmits an SIP message to an SIP proxy server that is part of a wired network. The SIP message is processed by the SIP proxy server, which communicates with another SIP proxy server through the network (e.g., the Internet). The second SIP proxy server then communicates with the destination node to which the call is directed.
In a wired environment (i.e., an environment in which the initiating node, SIP proxy servers, and destination node each communicates with the others via a wired network), there is little difficulty in using SIP and SDP messaging. This is due, in part, to the fact that the wired network connections have relatively high bandwidth and can easily communicate large amounts of data, such as SIP/SDP messages. This is also due, in part, to the fact that wired connections are so reliable that the associated error rate is very low and the SIP/SDP messages can be compressed into relatively few bits.
In a wireless environment, however, this is not the case. In a wireless environment, one or more of the initiating node, SIP proxy servers and destination node communicates with the others via a wireless connection. The communication of SIP/SDP messages in a wireless environment differs in two ways from the wired environment. First, generally speaking, the bandwidth of a wireless connection is substantially less than the bandwidth of a wired connection. As a result, the wireless connection cannot support the transmission of as much data as a wired connection. Second, because the wireless connection is generally less reliable (i.e., has a higher error rate) than a wired connection, it is not possible to transmit the SIP/SDP message in as compressed a form as can be done in a wired environment. Consequently, the SIP/SDP message transmitted over the wireless connection is ASCII-encoded to enable an acceptable error rate to be maintained.
SIP/SDP messages in a wireless environment may therefore comprise hundreds of bytes. They may even have more than a thousand bytes of data. It may therefore take minutes, or even tens of minutes, to complete transmission of the SIP/SDP message and to set up the voice-over-IP call, thereby presenting an unreasonable delay. Because the transmission of SIP/SDP messages over a wireless connection requires a much greater portion of the available bandwidth than in a wired connection, there is a much greater need for a means to make more efficient use of this bandwidth